Various curable liquid materials, such as silicones, urethanes and epoxy resins, have so far been widely used in the fields of not only rubber molding but also coating agents, potting agents, dipping agents and the like, making the best use of their characteristic features, for example their being liquid rubbers.
Among them, silicones are widely used as materials having high heat resistance, weathering resistance and reliability, among others, in various fields. In particular, those having a low viscosity and fluidity before curing are used as potting agents or like sealers in the field of electronics. Urethane resins, which are characterized by their high strength owing to the cohesive force of crosslinks and their water resistance and heat resistance owing to the main chain skeleton such as hydrogenated polybutadiene, are used not only in the field of electronics but also for waterproof coating. Epoxy resins, which are excellent in adhesiveness and heat resistance and are high strength materials, are used especially as sealers for semiconductor devices in the field of electronics and in other fields.
However, silicones have problems; for example, they show high permeability, have poor adhesiveness and are expensive. The problems which urethane resins have are, for example, that their electric characteristics lower upon moisture absorption and that they are inferior in heat resistance to silicones and epoxy resins. Epoxy resins are high in elasticity modulus, weak to cold and hot impact and inferior in insulating property to silicone resin. In the case of urethane resins and epoxy resins, the human toxicity of starting oligomers has become an issue.
On the other hand, curable materials comprising, as essential constituents, three components, namely an alkenyl-containing saturated hydrocarbon polymer (hereinafter referred to as main component), a hydrosilyl-containing curing agent and a hydrosilylation catalyst, have been developed as materials having other characteristic features than those of the above-mentioned materials. They are expected to be useful in various fields of application in which their high heat resistance, weathering resistance, water resistance and insulating properties, among others, can be made efficient use of.
However, the main component of this curable materials is a polymer, so that compositions based thereon, in an uncured state, may have a very high viscosity depending on the main component species. In particular when an isobutylene polymer is used as the main component, it is difficult to handle the isobutylene polymer as a liquid because of its high viscosity without applying some viscosity reducing technology, although the cured products obtained are excellent in various durability characteristics and, further, have low permeability and good vibration damping properties. Thus, for utilizing such curable compositions as potting agents or coating materials, it is essential to secure fluidity by some or other viscosity reducing technology. The most general technique as such viscosity reducing technology is the addition of a nonreactive diluent generally called plasticizer, for example an oil. In that case, however, while the viscosity is reduced by the addition of the plasticizer, the mechanical strength is markedly lowered and evaporation of the plasticizer at elevated temperatures causes heating loss and volume reduction of cured products, leading to marked changes in mechanical characteristics and, further, induces blooming of the plasticizer; reliability reduction thus results.
For solving such problems, a method has been proposed which comprises using in addition to the above three components, an organic compound containing, within the molecule, at least one alkenyl or alkynyl group capable of being hydrosilylated, combinedly as a fourth component. The designing of this method is that silicon atom-bound hydrogens of the curing agent, which are to serve as crosslinking sites, react with alkenyl-containing polymer, which constitute the main component, to form network chains and, at the same time, the fourth component, namely diluent itself is integrated into the crosslinked structure by reacting with such silicon atom-bound hydrogen atoms. For the formation of such crosslinked structure, it is essential that the network chain-forming polymer have crosslinking sites, namely bind to at least two curing agent molecules. The fourth component is integrated into the network structure by reacting with the remaining silicon atom-bound hydrogen atoms in the curing agent. Therefore, the addition amount of the fourth component is restricted by the number of silicon atom-bound hydrogen atoms in the curing agent. In other words, for incorporating the fourth component in a large amount, it is necessary to use a curing agent containing a large number of silicon atom-bound hydrogen atoms in each molecule. In this case, however, it is difficult to secure sufficient compatibility between the main component saturated hydrocarbon polymer and the curing agent. In particular, in compositions having a low viscosity and susceptible to phase separation, homogeneously cured products can hardly be obtained. To surmount these problems, improvements are to be brought about.
The present invention is to provide a curable composition which has a low viscosity in an uncured state and, in a wide viscosity range, gives a homogeneous rubber-like elastomer excellent in heat resistance, weathering resistance, water resistance, chemical resistance and other durability characteristics and in mechanical strength, and which could have never been attained by the prior art viscosity reducing technology.